A cluster state can be loosely defined as an entangled set of qubits arranged in a lattice. Breigel and Raussendorf strictly define a cluster state as “Let each lattice site be specified by a d-tuple of (positive or negative) integers a εZd. Each sight has 2 d neighboring sites. If occupied, they interact with the qubit a”. This implies a cluster state has interaction between all nearest neighbor qubits. In one dimension (d=1) this results in a linear chain of qubits, of arbitrary length with each qubit entangled with both of its nearest neighbors. All of the internal qubits will have two interactions while the edge qubits will have one. Such a one dimensional nearest neighbor cluster state has been shown to be amenable to several applications for computation presuming the cluster state is “long enough”.
Traditional generation of a cluster state consists of an optical table several meters on each side. On this table is a high power laser system such as a pulsed Ti:Sapphire laser. The pump beam is incident on a nonlinear material such as BBO, BiBO or PPKTP etc. The photons from the pump then have a small chance to undergo industry standard Spontaneous Nonlinear Parametric Down Conversion (SPDC) to create an entangled pair of photons, called signal and idler photons. Alternative means of photon generation are equally valid such as but not limited to four wave mixing (FWM). To create larger cluster states the pump passes through multiple nonlinear materials (a cascade configuration) or is reflected back onto the original material (a multi-pass configuration). These methods can create multiple simultaneous independent pairs of qubits. To create one large cluster state the pairs are sent through (i.e. acted on by) an entangling operation. Normally the industry standard two qubit entangling gate controlled phase gate (CPhase) or controlled Z gate (CZ) is used. The simplest and most efficient means of implementing the general CZ gate requires 3 bulk optical asymmetric beam splitters in a specific alignment. These operations are effectively performed in parallel with each qubit entering and exiting in its own mode. Once all the entangling operations are successfully completed the cluster state is fully constructed and an algorithm can be implemented as a sequence of single qubit rotations and measurements on each qubit in a predetermined sequence. Thus in the state of the art, linear cluster states are created from simultaneously generated qubits in parallel modes rather than from sequential qubits in a single mode. This is mainly due to the spontaneous nature of single photon sources. It is impossible to predict the time between two subsequent spontaneous events.
The present invention builds upon the periodic photons source of Mower and Englund (WO2013009946 A1) to create entanglement between sequential separable qubits delivered in a single mode and create a linear cluster state of sequential qubits which is output in a single mode. Such a device is of interest in and of itself for quantum computing. Applications include but are not limited to Measurement Based Quantum Computing (MBQC) implementation of the Deutsch-Jozsa algorithm on a four qubit chain, arbitrary single qubit rotations on a four qubit chain, quantum key distribution, quantum information, quantum metrology and quantum lithography.